American and British investigating and intelligence agencies are working to identify the English-speaking Islamic State jihadist who executed journalist James Foley on video, and then threatened to behead another U.S. journalist if the U.S. continues to launch airstrikes against the IS in northen Iraq.

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U.S. President Barack Obama said a relentless manhunt will now proceed to locate and arrest the people responsible for Foley's murder, including the terrorist who fronted for the IS and beheaded the American journalist.

In a report on The Guardian, sources are quoted as saying the man may be one of several British jihadists who have joined IS in Syria and Iraq and are now guarding foreign nationals, including journalists, being held in Raqqa, a town in northern Syria which is considered as a jihadist stronghold.

Sources in Syria said the man, who linguistics experts believed probably came from London's East End, could be one of the main negotiators in the release earlier this year of 11 IS hostages who were turned over to Turkish officials after payment of ransom.

The U.S. Federal Bureau of Investigation, MI5 and Scotland Yard's counter-terrorism unit were busy all Wednesday night looking for clues and consulting experts on the identity of the jihadist, who called himself John in the Foley execution video. Agents were analysing the propaganda footage, assisted by experts in forensic phonetics.

One former hostage, who was held for a year in Raqqa, told the Guardian he believed the militant is definitely British, is intelligent, educated and a devout believer in radical Islamic teachings. The former captive said the man could be one of three UK-born militants who were referred to as "the Beatles" by fellow hostages because of their nationality.

At the University of York, Professor Paul Kerswill, a linguistics expert, said he believed the man spoke in "multicultural London English" most commonly found in London's East End.

"He probably has a foreign language background but it sounds like multicultural London English, which is people from all kinds of backgrounds who mix in the East End, a new kind of cockney," Kerswill told the Guardian.

Dr Claire Hardaker, a linguistics experts at Lancaster University, also studied the clip and said the man is most likely from London.

"We're definitely looking at a British accent, from the south, and probably from London, Kent or Essex," says Dr. Hardaker.

ounter-terrorism sources said the man is likely to be already under the police radar, with previous criminal behaviour probably being scrutinized, the same was as other western jihadists who had gone overseas are being investigated.